The present invention relates in general to a method of and a device for feeding current in a subscriber's line in a telephone system having two independent constant-current sources arranged symmetrically to the subscriber's apparatus in the circuit so as to supply constant current independently of the length of the circuit.
From the German published patent application No. 2,556,157 a circuit arrangement of this kind is known in which current differences between the two branches of the subscriber's circuit are measured by means of a shunt and, in the case of different current values, a regulating device is employed to apply corresponding reference voltages to the constant-current sources so as to compensate for the current differences upwardly to a maximum current value. The two constant-current sources are constituted by a Darlington circuit of two transistors. One of the telephone lines is connected via the collector-emitter path of one of the transistors of the constant-current source to the positive pole of a central power source, such as a battery. The other telephone line is connected via the collector-emitter path of one of the transistors in the second constant-current source to the negative pole of the battery. The reference voltages for the constant-current sources are adjusted by means of voltage dividers. The regulating device includes two series-connected resistors bridging the circuit and acting as the measuring shut. In the occurrence of different current values measured between the two telephone lines of the circuit, the regulating device by suitably adjusting the reference voltages in at least one constant-current source restores the original current conditions, that is it provides, independently from the resistance of the subscriber's circuit, a uniform constant current in both branches of the telephone circuit. In order to prevent the generation of an excessively high circuit current, which may result for example by activation of a ground button at the subscriber's apparatus, a zener diode is connected to the common point of the series-connected shunting resistors. In this manner, the current through the subscriber's circuit is limited to a maximum permissible value.
The disadvantage of this prior-art energizing device can be seen in the fact that, in the case of a very low resistance of the telephone circuit to ground, no reduction of the power loss will occur. Moreover, it is also disadvantageous that the resulting non-symmetrical voltages in the telephone circuit influence the voltage regulation. The non-linear regulating characteristic cannot sufficiently eliminate this voltage asymmetry.